1. Field of the Invention
This invention relates generally to X-ray generators and is concerned more particularly with an X-ray generator having symmetrical beam egressing means.
2. Discussion of the Prior Art
An X-ray generator generally comprises a shielded housing having insulatingly mounted therein an X-ray tube, which may be cooled by a dielectric fluid flowing through the housing. The X-ray tube usually includes an evacuated envelope wherein an electron emitting cathode is disposed for beaming high energy electrons onto a sloped target surface of an axially spaced anode. Thus, X-rays are generated which emanate from the sloped target surface and radiate in a divergent beam through a radially aligned port in the housing.
Generally, the port is recessed and includes an X-ray transmissive window which is reentrant toward the focal spot area of the tube to reduce beam filtration. Because of its proximity to the highly positive anode of the tube, the window usually is made of an easily moldable dielectric material, which is provided with a flat surface adjacent the wall of the tube. As a result, it is found that a divergent X-ray beam passing through the window has a non-uniform distribution of X-ray intensity as a function of beam angle. The greater the divergent angle of the beam, the greater the absorption of X-rays adjacent the edges of the beam. This preferential angular absorption of X-rays in the divergent beam is particularly troublesome in certain diagnostic procedures where it is advantageous to have an incident X-ray beam cross-section which is substantially uniform. In computerized tomography, for example, it is desirable to detect, in an irradiated patient, an X-ray absorption difference of about one-half of one percent.
Therefore, it is advantageous and desirable to provide an X-ray generator with means for minimizing preferential angular absorption of X-rays in a divergent beam egressing from the generator.